Antibody (immunoglobulin, Ig) class switch recombination (CSR) is an essential mechanism for the diversification of humoral immune response through efficient generation of antibody isotypes that mediate elimination of pathogens. CSR is a programmed deletional recombination event between DNA double strand breaks in the Ig heavy chain gene locus (Igh). These DNA breaks are initiated by the mutagenic enzyme, activation-induced cytidine deaminase (AID), which preferentially deaminates the Igh genes but also exhibits ?off-target? activity in non-Ig genes including proto-oncogenes. Hence, DNA breaks initiated by AID in off-target regions of the genome lead to aberrant chromosome translocations between the Igh locus and oncogenes if not correctly repaired. Indeed, such genomic aberrations are a hallmark of B cell malignancies. Accordingly the goal of our research proposal is to characterize novel molecular mechanisms we have identified that regulate the repair of DNA lesions initiated by AID during CSR. We recently discovered that the telomeric protein TRF2, which is essential for protecting chromosome ends from DNA repair activity, is also needed for CSR. We hypothesize that TRF2 is essential for the activity of DNA repair proteins involved in the processing of DNA breaks initiated by AID during class switch recombination. Here we propose to extend our studies to identify and characterize the molecular mechanisms utilized by TRF2 to control the CSR via the following specific aims: 1. Determine the DNA repair proteins regulated by TRF2 during class switching. 2. Determine the molecular mechanisms used by TRF2 to control DNA repair during class switching. 3. Elucidate the mechanism by which TRF2 inhibits the formation of Igh-cmyc translocations. Taken together, these studies will provide specific details into a novel mechanism that regulates immune diversification in B cells and the maintenance of the genome stability during AID activation.